1. Field of the Invention
The present invention relates to a HID lamp having a high efficacy, and to its igniting device. The HID lamp is represented by a metal halide lamp, which is suitable as a white color and home-use light source and which has an arc tube including metal halides such as scandium, sodium and mercury, and filled with an inert gas such as argon.
2. Prior Art
Although a HID lamp, like a metal halide discharge lamp, has the advantages of a high efficacy and high color rendition, it has the significant defect that a hot-restrike or re-starting is very difficult, and disturbs a rapid on and off operation. One of the reasons for this is that the inner pressure in the arc tube remains very high when the lamp is extinguished after the lamp has been lit for several minutes following the initial ignition. In the conventional HID lamp, the initial ignition becomes easier at low voltages due to the establishment of a low pressure glow discharge between one of the main electrodes and a probe. This glow discharge is effective only at the beginning of the lit condition. However, once the glow discharge shifts to arc discharge, the inner temperature and pressure of the arc tube become very high due to the continued arc discharge. In such cases, the re-lighting of the lamp needs a hot re-strike or re-starting voltage of several tens of kilovolts for an arc tube having an electrode gap in the range of a few millimeters up to several ten millimeters.